This invention relates to a process for purifying a gaseous mixture by scrubbing with liquid nitrogen in a scrubbing column having two sections. Such processes can, for example, be employed for the purification of a gaseous mixture consisting essentially of hydrogen and containing impurities, e.g., carbon monoxide, methane and argon.
Such a process has been disclosed in German Pat. No. 947,711. In particular, in this conventional procedure a liquid nitrogen fraction poor in carbon monoxide is withdrawn by way of a side tap from an upper part of the bottom section of a scrubbing column. This results in an increase in the carbon monoxide concentration of the remaining scrubbing liquid withdrawn from the very bottom of the scrubbing column.
In the conventional process, a substantial excess of liquid nitrogen is utilized for safety reasons and on account of a lack of reliable calculations. This feature, on the one hand, permits the use of a shorter scrubbing column but, on the other hand, leads to a considerable CO dilution in the bottom of the scrubbing column. The side tap, for this reason, serves in the conventional process for removal of excess scrubbing nitrogen. This is also expressed in the fact that the carbon monoxide concentration in the fraction withdrawn via the side tap only amounts to maximally 5%.
The CO concentration in the bottom, and thus the calorific value of the resultant residual gas fraction, is maximized by the conventional process; however, if the CO is to be used for purposes other than heating, i.e. is converted, the CO must first be separated since the bottom fraction still contains the entire argon and methane that were contained in the feed gas. Such a purification of the CO is both complex and relatively expensive.